Antenna feed systems

ABSTRACT

An antenna feeder comprising a corrugated waveguide which affords on its inner surface a plurality of corrugated grooves in which the depths of successive grooves of the waveguide differ from each other so that the reflected impedance at the entrance of the grooves increases progressively from a low impedance afforded at the entrance of a groove at one end of the waveguide to a required higher impedance afforded at the entrance of a groove of the other end of the waveguide.

United States Patent Inventor Geoffrey 1111. Bryant ilford, England Appl. No. 875,164 Filed Nov. 10, 11969 Patented Nov. 2, 1971 Assignee The Plessey Company Limited lliord, England Priority Nov. 15, 1968 Great Britain 541427/68 ANTENNA IFlElED SYSTEMS 7 Cinirns, 4 Drawing ll igs.

U.S. til 3d3/772, 333/34 Int. Cl] 11013113702, H03n 7/38, HOlg 13/06 Field of Search 333/33-34, 35. 95, 98; 343/772-775 References Cited UNITED STATES PATENTS 2,395,560 2/1946 Llewellyn 2,567,748 9/1951 White 333/95 X 2,573,012 10/1951 Gutton eta] 333/95 X 2,623,121 12/1952 Loveridge 333/95 2,641,731 6/1953 Lines 333/95 X 2,659,817 11/1953 Cutler 333/34 OTHER REFERENCES Elliot, On the Theory of Corrugated Plane Surfaces in Transactions of the IRE Professional Group on Antennas and Propagation, V01.AP-2 No. 2, Apr. 1954, pp. 71- 76 Primary Exnminer- Herman Karl Saalbach Assistant Examiner- Marvin Nussbaum At!0rneyScrivener Parker Scrivener and Clarke ANTENNA FEED SYSTEMS This invention relates to antennafeed systems and relates more specifically to antenna feeder arrangements incorporating transversely corrugated waveguide feeds.

An antenna feeder is normally positioned at the focal point of a transmit/receive aerial and is effective for directing the transmitted signal to the aerial proper in order that it may be radiated as well as for collecting the received information from the aerial proper for feeding to the normal detection circuits.

The aims of good antenna feed design are a. control of beam shape,

b. reduction of side-lobe ievel to avoid spillover,

c. elimination of back-lobe, I

d. production of a well-defined phase center the position of which is independent of space angle,

e. equal beam width in all space planes perpendicular to the beam axis,

f. broad bandwidth, limited ideally by feeder waveguide dimensions,

g. compactness,

h. good tracking performance in a monopulse application,

i. good match.

Current methods of satisfying some of these conditions include the use of waveguide horns with longitudinal or transverse fins. One form of antenna feeder has been developed and is described in a report by Alan F. Kay entitled "The Scaler Feed" prepared for the Air Force Cambridge Research Laboratories, Office of Aerospace Research, U.S.A.F., Bedford, Mass, copies of which are available from US. Department of Commerce, Office of Technical Services, Washington 25, DC. The feeder described meets conditions (a) to (e) above but appears to be only partially successful so far as conditions (f) to (i) are concerned. The claim of wide band operation of the feeder has not been confirmed in practice; tracking has only a narrow bandwidth and the match is poor except over bandwidths of the order of 7 percent. Longitudinally finned horns are well known to give good beamwidth equality, but have highside lobes and poor match over more than 1 or 2 percent bandwidth.

it is an object of the present invention to provide an antenna feeder arrangement incorporating transversely corrugated waveguide feeds which optimizes conditions (a) to (i) above.

According to the present invention an antenna feeder arrangement comprises a corrugated waveguide which affords on its inner surface a plurality of transverse grooves, in which the depths of successive grooves of the waveguide differ from each other so that the reflected impedance at the entrance of the grooves increases from a low impedance afforded at the entrance ofa groove at one end of the waveguide to a required higher impedance afforded at the entrance of a groove at the other end of the waveguide.

in a preferred arrangement according to the invention it will be arranged that the depth of a groove at one end of the waveguide is substantially equal to one-half of the wavelength of the dominant mode of the groove and the depth of a groove at the other end of the waveguide will be substantially equal to one-quarter of the wavelength of the dominant mode of the groove.

in one arrangement it may be arranged that the inner surface of the waveguide is substantially parallel to the longitudinal axis of the waveguide in which case the antenna feeder according to the present invention may conveniently be used to connect a plain waveguide to a flared corrugated waveguide having grooves of depth substantially equal to one-quarter of the wavelength of the dominant mode of the groove.

in a further arrangement it may be arranged that the inner surface of the antenna feeder according to the present invention is itself flared in the longitudinal direction from said one end to said other end.

in applying the antenna feeder according to the present invention to waveguides having square or rectangular cross sections, there may be provided means inserted in the grooves of the waveguide at the corners thereof to reduce the efi'ect of spurious modes set up due to the varying depths existing at said comers.

Some exemplary embodiments of the invention will now be described with reference to the accompanying drawing, in which:

FIG. 1 is a cross-sectional side view of an antenna feeder according to the present invention;

FIGS. 2a and 2b are side and end views, respectively of an alternative form of antenna feeder according to the present invention, and;

FIG. 3 is a cross-sectional side view of a transversely corrugated waveguide incorporating corner pieces according to the present invention.. H g 4 A in known antenna feed systems using transversely corrugated waveguides, the corrugated waveguide, whether it be flared or'not, is fed from a plain waveguide, i.e., one having no corrugations. The depth of the corrugations or grooves of the feeder is normally in the order of )rg/4 where kg is the wavelength of the dominant mode set up in the groove and each groove thus forms a short-circuited waveguide whose length is approximately )tg/4. Now, it is known from waveguide theory that the impedance reflected across the input terminals of a waveguide which is M4 long is infinitely high, i.e., an open circuit, and so the impedance reflected at the entrance to each groove of the corrugated waveguide is also very high. The effect of this is that when awave travelling along the wall of a plain waveguide meets the first groove of the corrugated waveguide then the impedance of its path changes from a very low value to a very high value and the effect of this abrupt change is to set up undesirable modes in the feeder which degrades'itsperformance.

This is overcome according to the present invention by arranging that the transition from a plain waveguide to a corrugated waveguide having grooves of )tg/4 takes place gradually. This is achieved by arranging that the first groove of a corrugated waveguide that a wave travelling along a plain waveguide meets presents an impedance at its entrance which is effectively very low e.g., a short circuit. This can be achieved for instance by arranging that the depth of the first groove is Ag/Z. The depths of the succeeding grooves are gradually decreased so that the impedance presented at their openings gradually increases until the groove depth is at the required value of kg/4. This decrease in the depth of the grooves from kg/Z to Ag/4 may take place for example over a distance of 2kg and there may be provided, for example, in this length 10 grooves of succeedingly decreasing depth.

In FlG. l of the accompanying drawings there is shown an application of this technique to a broadband sum channel sealer feeder. The feeder shown consists of a plain, square cross section waveguide 1 which is joined to a flared, square cross section, corrugated extension 2 by means of a square cross section, corrugated matching section 3. The extension 2 is flared at an angle 0 to the longitudinal axis of the feeder and the grooves 4 afforded by the corrugations 5 have a depth of )tg/4. The matching section 2 is provided with tapered corrugations 6 such that the groove nearest the plain waveguide 1 has a depth of kg/Z and the groove nearest the flared extension 2 has a depth of kg/4 i.e., the same depth as the grooves 4 of the flared extensionZ.

Turning now to FIGS. 2a and 2b of the accompanying drawings, there is shown an antenna feeder suitable for use as a broadband tracking feed. In this application the feed is. required to transmit and receive circularly polarized waves, the transmit and receive channels each having a 7 percent bandwidth. The receiver channel would normally incorporate monopulse tracking with sum and difference circuits but for the sake of simplicity, the feeder only will be described. The feeder shown in FIGS. 2a and 2b ofthe accompanying drawing consists of a plain square cross section waveguide 7 which is subdivided into four subsidiary waveguides 8 (these being used for monopulse tracking) and the waveguide 7 is joined to transversely corrugated end section 9'. The corrugated end section 9 is provided with grooves 10 of succeedingly decreasing depth such that the groove 10 nearest the plain waveguide 7 has a depth of approximately hg/Z and the groove 10 farthest from the plain waveguide 7 has a depth of approximately Ag/4. When such an arrangement is caused to radiate in free space the field patterns are typical of cosinusoidal aperture distributions in the electric (E) and magnetic (H) planes so that circular polarization may be used. The inner surface of the end section 9 is also provided with a slight flare of angle which has the effect of controlling the radiation while preserving equal beam widths but makes them less dependent upon frequency.

lt has been found that antenna feeders such as those described afford equal E and H plane beamwidths; provide a good match to the input feed; have a 15 percent bandwidth in the 7 to 8 cHz. range and enable small size apertures i.e., less than 2 A to be used.

In the foregoing embodiments waveguides of square cross section have been used, but it should be understood that the same principles may be applied to waveguides having nonsquare cross sections, e.g., rectangular or circular. It will also be understood that the inner surface of the waveguide may be of constant cross section or flared dependent upon the particular application.

In feeders such as those described in which square cross section waveguides are used, it has been found that undesirable modes which have the effect, among other things, of limiting the bandwidth are set up in the waveguide due to the varying depths of the grooves at the corners of the waveguides. It has been found that the effect of these spurious modes can be substantially reduced by filling in the grooves in the vicinity of the corners of the waveguide with shaped corner pieces. This is shown in FIG. 3 of the drawing which is a cross-sectional view of a square cross section waveguide 11 in which the sides of a groove of the waveguide is shown at 12 and in which corner pieces 13 have been inserted in the four corners of the waveguide. In one particular arrangement it has been found that the optimum shape of the corner pieces is a square in which the inside corner has been chamfered. However, the shape of the corner pieces may be made to suit any particular application.

What we claim is:

1. An antenna feeder arrangement comprising a corrugated waveguide which affords on its inner surface a plurality of transverse grooves, in which the depth of successive grooves of the waveguide decreases from a depth substantially corresponding to one-half of the wavelength of the dominant mode of the groove at the entrance of the waveguide to a depth substantially corresponding to one-quarter of the wavelength of the dominant mode of the groove at the exit of the waveguide.

2. An arrangement as claimed in claim 1, in which the inner surface of the waveguide is substantially parallel to the longitudinal axis of the waveguide.

3. An arrangement as claimed in claim 1, in which the inner surface of the waveguide is flared in the longitudinal direction.

4. An arrangement as claimed in claim 1 in which the corrugated waveguide is of rectangular cross section.

5. An arrangement as claimed in claim 4, comprising means for reducing the effect of spurious modes set up in the waveguide due to the varying depth of the grooves at the corners of the waveguide.

6. An arrangement as claimed in claim 5, in which corner pieces are inserted in the grooves at the corners of the waveguide.

7. An arrangement as claimed in claim 6, in which the corner pieces are of substantially square cross section, the inner corner of each corner piece being chamfered. 

1. An antenna feeder arrangement comprising a corrugated waveguide which affords on its inner surface a plurality of transverse grooves, in which the depth of successive grooves of the waveguide decreases from a depth substantially corresponding to one-half of the wavelength of the dominant mode of the groove at the entrance of the waveguide to a depth substantially corresponding to one-quarter of the wavelength of the dominant mode of the groove at the exit of the waveguide.
 2. An arrangement as claimed in claim 1, in which the inner surface of the waveguide is substantially parallel to the longitudinal axis of the waveguide.
 3. An arrangement as claimed in claim 1, in which the inner surface of the waveguide is flared in the longitudinal direction.
 4. An arrangement as claimed in claim 1 in which the corrugated waveguide is of rectangular cross section.
 5. An arrangement as claimed in claim 4, comprising means for reducing the effect of spurious modes set up in the waveguide due to the varying depth of the grooves at the corners of the waveguide.
 6. An arrangement as claimed in claim 5, in which corner pieces are inserted in the grooves at the corners of the waveguide.
 7. An arrangement as claimed in claim 6, in which the corner pieces are of substantially square cross section, the inner corner of each corner piece being chamfered. 